Multi-color gradient ceramics produced by aqueous tape casting

OBJECTIVE: Functionally graded materials find currently an increasing interest, especially in ceramic field, thus broadening the ranges of applications, e.g. for separation membranes, tribological applications, thermal barriers or just esthetic purposes. A general procedure was developed to produce stacks of hundreds of layers with different material properties in such controlled arrangement that smooth macroscopic gradients through the whole block are achieved after firing. As a case study we demonstrate the possibility to create multiple color gradient within ceramic blocks and to machine those to specific shapes. MATERIALS & METHODS: Ceramic powders, cellulose and vinyl acetate‐based aqueous slurries were used to produce flexible green tapes with thickness ranging from 60 to 200 microns. The fabrication of ceramic blocks followed the stacking up of color tapes at room temperature using low‐pressure moldless compaction. Subsequently, the blocks were pre‐sintered, and then CNC machined in various shapes and sintered to full density. Scanning electron microscopy, stereo microscopy and UV‐Vis spectrophotometry were used to analyze the interior microstructure and gradient color zones. RESULTS: Different prefabricated blocks were shaped by CNC milling and subsequently fired. All ceramics specimens, depending on individual material, achieved relative densities around 99 %. The results from CIE Lab color space coordinates that were calculated from spectrophotometer’s reflectance spectra showed high degrees of linearity or reflected the applied material pattern. Results of hardness tests and bending strength were found to be in close match with literature data. There was no significant difference among the perpendicular and parallel layers orientation in stacked blocks. CONCLUSIONS: The developed process allows production of functionally graded ceramic materials. As an example, the study shows smooth transitions between color boundaries within ceramic blocks. It can be concluded that process does not negatively affect the mechanical properties and density of ceramic bodies. Furthermore, it provides a more flexible way of creating gradient zone through the ceramics. This makes it a very attractive method, especially for dental applications, but also for other esthetic demands e.g. jewelry and watch industry